The local 36-year-old, a physician, has a thriving practice and two young children. She's active in the community, volunteering at food pantries and coaching a local school's cross-country team. Brady herself is an avid runner, having completed four marathons and countless shorter races, from 5Ks to half-marathons.

Still, she says, there's one thing missing—and it haunts her, day and night.

"I've been a runner for seven years," she says, "and I have never crushed a race. Not once."

Brady is quick to acknowledge her running accomplishments—her personal record for the marathon is a very respectable 3 hours and 17 minutes—and to express gratitude for a long, healthy career as a runner. She's equally open in pointing to races that she has "nailed" and PRs that she has "smashed" or "shattered."

At this point, I’d be happy just to crush a 5K. I don’t care what race I crush, as long as I crush it.

But after seven years, she has yet to "crush" a race. Of any distance.

"At this point," she says, laughing, "I'd be happy just to crush a 5K. I don't care what race I crush, as long as I crush it."

Experts say Brady's experience is not uncommon. As many as 15% of runners will never crush a race. Often a genetic defect may be to blame—in simple terms, some of us are just incapable of crushing a race. Other times, it comes down to bad luck.

"There are so many variables that factor into crushing a race," explains Sam Franklin, Ph.D., a professor of exercise science at the University of Minnesota. "Most of us, if we keep at it long enough, will hit that combination at least once. A few of us simply won't."

Whatever the reason, Brady says, she finds it hard to cope. She stopped using Facebook about a year ago, finding her news feed too much to bear.

"It was full of friends' race reports and finishing times," she says. "They were all crushing it. I was happy for them, but every single post hit me like a dagger."

Still, Brady says she's hanging in there and trying to stay positive.

"I'm hopeful," she says. "Every time I toe the line, I think, This could be the race I crush. I just keep telling myself that running isn't about self-pity. It's about perseverance."

Mark Remy's DumbRunner.com

DumbRunner.com is a website for runners who enjoy laughter and pie, from longtime writer and marathoner Mark Remy. Dumb Runner offers humor, insight, and 100% B.S.-free advice for runners of all abilities.